As feature size in the IC industry continues to decrease, practical and reliable low dielectric constant materials become increasingly important.
One dielectric material which is often used is silicon dioxide. Silicon dioxide is typically selected because it has good physical and electric properties. For example, silicon dioxide has good mechanical stability at elevated temperatures and typically has a dielectric constant in the range of 4 to 5. However, as the art moves towards faster and lower power applications, it is desirable to produce a dielectric material with a dielectric constant of 3 or less.
One promising dielectric material with a dielectric constant less than 3 is Parylene AF4. Parylene AF4 is a parylene coating derived from di-para-xylylene ("Parylene AF4 Dimer," available from Specialty Coating Systems, Inc. located in Indianapolis, Ind.). As set forth in Beach et al., U.S. Pat. No. 5,538,758, (hereinafter Beach et al.), incorporated herein by reference in its entirety, thin film Parylene AF4 is obtained by a well known process in which the Parylene AF4 dimer is vaporized, pyrolized and then fed into a deposition chamber wherein the Parylene AF4 is deposited on the substrate.
To vaporize the Parylene AF4 dimer, Beach et al. teach that the powdered Parylene AF4 dimer is placed in a dimer crucible and heat transfer receptacle. Electrical heating elements are then used to heat the heat transfer receptacle thereby vaporizing the Parylene AF4 dimer powder. However, once vaporized, the Parylene AF4 dimer vapor has a tendency to condense as solid Parylene AF4 dimer on the inner walls of several areas of the apparatus. This undesirable condensation of Parylene AF4 dimer vapor reduces the cost effectiveness of forming the Parylene AF4 thin film and also contaminates the apparatus. Accordingly, the art needs a method of vaporizing Parylene AF4 dimer which avoids the waste and expense resulting from Parylene AF4 dimer vapor condensation and the associated system contamination.
To avoid the undesirable accumulation of solid Parylene AF4 dimer, Beach et al. teach providing independent heating elements to heat the various parts of the apparatus which contact the Parylene AF4 dimer vapor. However, these independent heating elements unevenly heat the various parts of the apparatus resulting in temperature differentials and localized cold spots. As a result, condensation of the Parylene AF4 dimer vapor inevitably occurs.
Beach et al. teach that the heat transfer receptacle can be cooled to reduce or quench vaporization of the Parylene AF4 dimer powder. However, as set forth above, solid Parylene AF4 dimer powder accumulates on various parts of the apparatus, and these parts are heated. Consequently, even when the heat transfer receptacle is cooled, significant Parylene AF4 dimer powder vaporization continues from the other heated parts of the apparatus. Further, even when the heat transfer receptacle is heated to produce Parylene AF4 dimer vapor, the contribution from the other heated parts of the apparatus to the total Parylene AF4 dimer vapor flow is unpredictable. Thus, the apparatus of Beach et al. is not well suited for producing a stable and controllable flow of Parylene AF4 dimer vapor.
Uncontrollability of the Parylene AF4 dimer vapor flow in turn results in uncontrollability of the deposited thin film Parylene AF4 thickness. For example, if the Parylene AF4 dimer vapor flow is higher than expected, then the resulting Parylene AF4 film will be too thick. Thus, the apparatus of Beach et al. is not well suited for semiconductor type process applications where repeatable controlled deposition of thin films is required. Accordingly, the art needs a method of producing a stable and controllable flow of Parylene AF4 dimer vapor. The method should also be relatively simple to allow implementation in a manufacturing environment.